Octreotide is a highly potent and pharmacologically selective analog of somatostatin. It inhibits growth hormone for long duration and is thereof indicated for acromegaly to control and reduce the plasma level of growth hormone. The presence of D-Phe at the N-terminal and an amino alcohol at the C-terminal, along with D-Tryptophan and a cyclic structure makes it very resistant to metabolic degradation.
Octreotide comprises 8 amino acids which has the following structural formula:
wherein sulphur atoms of the Cys at the position 2 and of the Cys at the position 7 are mono-cyclic to form an —S—S— bridge.
A considerable number of known, naturally occurring small and medium-sized cyclic peptides as well as some of their artificial derivatives and analogs possessing desirable pharmacological properties have been synthesized. However, wider medical use is often hampered due to complexity of their synthesis and purification. Therefore, improved methods for making these compounds in simple, lesser steps and at lesser cost are desirable and this is the felt need of the industry and the mankind.
Conventional synthesis of octreotide may be divided into two main approaches, direct solid-phase synthesis and liquid-phase synthesis. Solution phase synthesis has been described by Bauer et al., (Sandoz) (Eur. Pat. Appl. 29,579 and U.S. Pat. No. 4,395,403). The process comprises: removing protected group from peptide; linking together by an amide bond two peptide unit; converting a function group at the N- or C-terminal; oxidizing a straight chain polypeptide by boron tristrifluoroacetate. This process involves a time-consuming, multi-step synthesis, and it is difficult to separate octreotide from the reaction mixtures since all the synthesis steps are carried out in liquid phase. Another solution phase approach described by Chaturvedi, et al., (Wockhardt) in U.S. Pat. No. 6,987,167 and EP 1506219 A, claims the cyclization of partially deprotected octreotide in the solution phase using iodine under conditions and for a time sufficient to form the octreotide.
Synthesis in solid phase have been described subsequently (Mergler et al., Alsina et al., Neugebauer). The above prior art for solid phase peptide synthesis cites the octapeptide formation, by starting the synthesis from the threoninol residue which makes it mandatory to protect this residue. Mergler et al., (Peptides: Chemistry and Biology. Proceedings of the 12th American Peptide Symposium. Smith, J. A. And Rivier J. E. Eds ESCOM, Leiden, Poster 292 Presentation, (1991)) describes a synthetic process, using an aminoethyl resin upon which the Threoninol residue is incorporated with the two alcohol functions protected in acetal form The synthesis is carried out following an Fmoc/tBu protection scheme, forming the disulphide bridge on resin by oxidation of the thiol groups of the previously deprotected cysteine residues and releasing and deprotecting the peptide with a 20% mixture of TFA/DCM.
In early 1997, Alsina J. et al. (Alsina J., Chiva C., Ortiz M., Rabanal F., Giralt E., and Albericio F., Tetrahedron Letters, 38, 883-886, 1997) described the incorporation, on active carbonate resins, of a Threoninol residue with the amino group protected by the Boc group and the side chain protected by a Bzl group. The synthesis was then continued by Boc/Bzl strategy. Formation of the disulfide bridge was carried out directly on resin using iodine and the peptide was cleaved from the resin and its side chain protecting groups were simultaneously removed with HF/anisole 9/1. At the final stage the formyl group was removed with a piperidine/DMF solution.
Neugebauer (Neugebauer W., Lefevre M. R., Laprise R, Escher E., Peptides: Chemistry, Structure and Biology, p 1017, Marshal G. R. And Rivier J. E. Eds. ESCOM.Leiden (1990) described a linear synthesis with a yield of only 7%.
Edwards et al., (Edwards B. W., Fields C. G., Anderson C. J., Pajeau T. S., Welch M. J., Fields G. B., J. Med. Chem. 37, 3749-3757 (1994) carried out another another solid-phase type approximation; they synthesized step-by-step on the resin, the peptide D-Phe-Cys(Acm)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(tBu)-Cys(Acm)-HMP-Resin. Next they proceeded to form the disulfide on resin and then release the peptide from the resin by means of aminolysis with threoninol, with obtaining a total yield of only 14%.
The solid phase synthesis described by Yao-Tsung Hsieh et. al., in U.S. Pat. No. 6,476,186 involves the synthesis of octreotide by using Thr(ol)(tBu)-2Cl-trityl resin as starting material followed by the cleavage of the straight chain peptide from the resin by using a strong acid and the formation of the intra-molecular disulfide bond on the completely deprotected octreotide by oxidation using charcoal catalyst and a higher yield of >70%.
Another solid phase synthesis described by Berta Ponsati et.al (Lipotec) in U.S. Pat No. 6,346,601 and EP 0953577 B involve the coupling of threoninol on the protected heptapeptide in solution, after a selective acid cleavage from the chlorotrityl resin without affecting the peptide side-chain protecting groups.
A hybrid solid phase-liquid phase method for synthesis of octreotide described by Iarov et al., (Dalton Chemical Laboratories) in WO 2005087794 wherein the method comprises liquid phase condensation of two or three peptide blocks in which at least one peptide block is synthesized by solid-phase method.
EP 1511761 B1 involves cyclization on the semi-protected linear peptide wherein one of the cysteine residue is protected with an orthogonal protecting group.
The radioactive isotope labeling of octreotide by the coupling of bifunctional chelating agents like DTPA or DOTA to the peptide was described by Te-Wei Lee et al., in U.S. Pat. No. 5,889,146 (Inst. of Nuclear Energy Research)
The method for cyclization of linear vapreotide by means of intramolecular cysteine formation has been described by Quattrini et. al., (Lonza AG) in WO 2006048144, wherein the process involves the synthesis of linear vapreotide peptide on Sieber-resin (from Novabiochem) by Fmoc standard groups, wherein the side chain protecting groups are D or L-Trp(Boc), Cys(Trt), Lys(Boc), Tyr(tBu). The protected peptide is cleaved off in 5% TFA in dichloromethane and then globally deprotected by acidolysis in a cleavage mix of 300 equivalents of concentrated TFA, 12 equivalents of Dithiothreitol, 12 equivalents of Dichloromethane, 50 equivalents of water for 1 hour at room temperature. The Boc groups are removed. The product was subjected to charcoal method using trace amounts of activated, powdered charcoal wherein a concentration of the linear cysteinyl peptide of 50 mg/ml (1 eq.) in DMF in the presence of 1 eq. Diisopropyl-ethyl-amine and that additionally air was sparged at low pressure into the liquid under stirring. After 15-20 hrs, 100% conversion was achieved with 84% (w/w) analytical yield of 79% vapreotide.
The formation of intramolecular disulphide formation in a polypeptide by reacting with hydrogen peroxide has been described by Mineo Niwa et al. (Fujisawa Pharmaceutical Co.) in U.S. Pat. No. 5,102,985 wherein the reaction is to be carried out at a pH of about 6 to 11, wherein the molar ratio of H2O2 to polypeptide is within the range of 1:1 to 100:1.
The above cited prior art mainly carries out the cyclization of the peptide on the resin or on partially protected or protected peptides. The use of partial or minimal protecting group strategies and improvement in the activation methods have considerable effect on limitations of poor solubility and possible danger of racemization due to the overactivation of carboxyl groups. However, these approaches do not overcome the problem of the poor coupling efficiency between large peptide segments, because of the intrinsic difficulty of obtaining effective molar concentrations for high molecular weight molecules.